See simple effect of lighting on complicated material?
Where the output of the normal node is connected to the mix factor you can add a ColorRamp node. By bringing the white and black points close together you will reduce the amount of blending between the two. The location of the points will determine where the change happens.

You can also try different interpolations with B-Spline being a slow transition and constant being a very sudden change.



You can try using the material option for viewport shading to get a fast preview, or you could use simple diffuse nodes like shown here until you get the position right and then connect the rest of your nodes into the mix shader once you have it.